CLIAMS:We Claim:
1. A process for making the briquettes from the gas cleaning plant (GCP) sludge of the LD steel making, the process comprising-
- drying the gas cleaning plant (GCP) sludge in an open atmosphere for a period of at least 12-15 days
- sieving the dried sludge for removal of extraneous matter
- adding the lime and molasses to the sieved sludge
- mixing the lime and molasses in a mixer for a desired period to increase the binding strength of the sludge.
- passing the mixture to the briquette making machines to make briquettes
- curing the briquettes for 3-5 days to attain the desired strength

2. A process as claimed in claim 1, wherein the size of sieved particles of the sludge is 3-5mm.

3. A process as claimed in claim 1, wherein the mixture of sludge, lime and molasses is mixed in mixer for 30-40 minutes.

4. A process as claimed in claim 1, wherein the quantity of lime added to the sieved sludge is 3-5% w/w of the sludge.

5. A process as claimed in claim 1, wherein the quantity of molasses added to the sieved sludge is 4-6% w/w of the sludge.

6. A process as claimed in claim 1, wherein the maximum pressure applied in the briquette making machine for making the briquettes is about 12.411 Mega Pascal (1800 pounds per square inch). Required briquetting pressure is 9.8 - 10.8 MPa.

7. The briquettes prepared by the process as claimed in claim 1, wherein the FeO; CaO; MgO; SiO2; P contents are 60-65% w/w; 10-14% w/w; 2-5% w/w; 3-5% w/w; 0.1-0.2% w/w respectively.

8. The briquettes prepared by the process as claimed in claim 1, wherein moisture of the briquettes is less than 1.5 w/w.

9. The briquettes as claimed in claim 7, wherein the shatter index (+10mm) is 90%-98%.

10. The briquettes as claimed in claim 7, wherein the dimension along major axis is 45-55 mm.

11. The briquettes as claimed in claim 7, wherein the dimension along minor axis is 22-27 mm.

12. The briquettes as claimed in claim 7, wherein the thickness of briquettes is 19-21mm.

13. The briquettes as claimed in claim 7 whenever used in replacement of iron ore in Linz –Donawitz (LD) steel making process as a coolant.

14. The briquettes as claimed in claim 7, wherein the replacement ratio is 1:1 to 2:3 according to requirement.
,TagSPECI:The invention relates to a process of making the briquettes from the gas cleaning plant (GCP) sludge generated in the LD steel making process and the briquettes made thereof. These briquettes can be used as a replacement of the iron ore in Linz – Donawitz (LD) steelmaking process for temperature control purpose.

Background
The Gas Cleaning Plant (GCP) sludge is a byproduct generated in steel making process. Steel is made in LD converters by blowing pure oxygen, which converts the carbon present in pig iron into carbon mono-oxide. Oxygen is blown through lances into liquid iron. During oxygen blowing process, a large amount of fumes and gases are generated, which contain fine particles of the charged materials and carbon monoxide (CO) gas.

This dust laden gas is cleaned before utilization as a fuel. The gas is cleaned using water and stripped of suspended solids in gas cleaning plant (GCP).

The outlet water from GCP is sent to treatment plant (TP) where solid particles are separated from CGP effluent water by sedimentation. The sediment is called the GCP sludge.

As this gas cleaning plant (GCP) sludge is produced in considerable amount its removal is also a concern to the Steel Industries. If it is dumped at a place for a long time it may cause the environmental problems as well as the scarcity of space for storage.

Further very good amount of the iron present in the gas cleaning plant (GCP) sludge hence the recycling of the iron is also necessary and beneficial to the steel manufacturers.

Keeping into the view all the above aspects the present invention describes the method for the recycling the gas cleaning plant (GCP) sludge.

Prior Art

There is a substantial need for the pollution control in India as well as in whole world. There is considerable amount of gas cleaning plant (GCP) generated from the LD process in steel industries as in our country.

Various practices are suggested time to time which have been tried to utilize and recycle the gas cleaning plant (GCP) sludge in various forms from land filling to and slag for road bed and cement production. Most common practice is to recycle in sinter making units of integrated steel plants but that has limitations.

European Patent EP1448492 relates to an additive mixture for cement clinker production, consisting of a mixture of slag formed in the course of producing pig-iron and steel and hydrocarbon based wastes deriving form mineral oil industry or materials containing such wastes, of a maximum grain size of 20 mm, in a quantity and proportion adaptable to the raw material matrix of cement clinker. Conditioning agents, for example packaging materials, wood favourably polluted sawdust or cuttings, paper or cardboard, may be added. The additive mixture is fed in the cement clinker burning furnace at the section of 1.050-1.150 DEG C and added to furnace flour of a temperature of 800-850 DEG C.
International Publication WO9845485 describes a process for dehydrating wet BOF scrubber sludge to produce a steelmaking revert having an improved flow rate when handled in a recycle stream. Wet sludge is combined with hot BOF slag to provide a slag/sludge mixture. The wet sludge causes the mixture to have a moisture content greater than 10 % water by weight, and the hot slag, having a temperature below the molten liquid state, vaporizes the water in the mixture and reduces the moisture content to about 4 % water by weight or less. The dehydrated mixture has improved flow rate properties when it is recycled as a steelmaking revert.

US Patent No. 4,432,666 describes a method of depositing and ultimately disposing of special waste containing constituents which under environmental conditions can be eluted and emitted, by mixing the special waste with setting water and with a binder selected from the group consisting of calcium hydroxide, gypsum hardening on addition of water, waste gypsum, cement and cement-like products.

Description

It is a common practice to add sized iron ore during the LD steel making process to control bath temperature. Iron ore acts as coolant in the LD process.

Supersonic jet of oxygen is blown into the furnace to remove carbon and silicon in the molten iron mainly by oxidation of these impurities. The basic oxide furnace is fed with fluxes to remove impurities.

When the oxygen is blown in the furnace lot of gas mostly Carbon Mono Oxide (CO) is generated. This gas is recovered and used as a fuel in steel plants.

The gas comes out of LD vessel along with a lot of solid particles. The gas containing the solid particles is passed through the water to clean.

In treatment plants, the solid particles are recovered as sludge from the outlet water of gas cleaning plant (GCP) .

According to an embodiment of the invention the sludge generated is allowed to dry in open atmosphere for about a period of 13-18 days. Alternatively it can be dried with an electrical or gas fired drier. The dried sludge is transported with dumpers to the briquette making site or briquetting area.

Dried gas cleaning plant (GCP) sludge contains mainly iron, calcium oxide, and magnesium oxide, besides alumina, silica, basic oxides and zinc. The percentage of major constituents of the sludge is as per the following table-
Composition of dried sludge (% weight)
Total Fe CaO MgO SiO2 P
64-70 8-12 1-4 3-5 0.2-0.3

According to another embodiment of the invention, the dried sludge is sieved to <5 mm particle size to remove unwanted material. After sieving the sludge, the sludge is mixed with the hydrated lime. The quantity of the lime is approximately 3-5% of the sludge. The molasses about the 4-6 % of the sludge is also mixed with the sludge to increase binding strength of the sludge.

The mixing of these materials is done in a mixer for a sufficient period (30-40 minutes). The mixed material is then transported to bunkers through the conveyers. The mixer feed rate is controlled using bunker gates.

According to further embodiment of the invention, the material mixed with binders is passed through screw conveyor to give uniform feed. The screw conveyor feeds the material to the briquetting rolls. The speed and gap between the rolls are adjusted according to quality of output.

Once the feed rate, speed of rolls and pressure applied are adjusted the process is ready for continuous production. Maximum pressure applied of the briquette machines is 12.411 Mega Pascal (MPa) (1800 pounds per square inch) in the present invention. Required briquetting pressure is 9.8 - 10.8 MPa. The dimensions of the briquettes formed are 45-55 mm along major axis, 22-27 mm along minor axis, 19-21 mm in thickness. Briquettes are stored in a specified area and allowed to cure for minimum three days to attain strength and decrease moisture content.

The invention will be described with reference to following example without restricting the scope of the invention.

Example:
Process for making the briquettes from the gas cleaning plant (GCP) sludge of the LD steel making, the process comprises
- drying the gas cleaning plant (GCP) sludge in an open atmosphere for a period of at least 13 days
- sieving the dried sludge of the particle size -5mm for removal of extraneous matter
- adding the lime 3.5 4% and molasses 4.5 5% (w/w of dry sludge) to the sieved sludge
- mixing the lime and molasses in a mixer for 35 minutes to increase the binding strength of the sludge.
- passing the mixture to the briquette making machines to make briquettes
- curing the briquettes for 4 days to attain strength of shatter index of 95%

The final moisture content of briquettes is critical for proper handling and charging. Too low moisture in briquettes can result in dust generation and too high may cause plugging of hoppers and bins. Presence of moisture can create following problems:
(i) Causing explosion in converter vessel due to the presence of Oxygen (O2) and high temperature generation of Hydrogen (H2)
(ii) Disintegration of briquettes will take place due to; softening; and
(iii) Wet sludge/briquette not passing smoothly through feeders and chute and resulting in lump formation in the bunker.

The moisture less than 1.5% is optimum to avoid the above problems. The moisture level (> 7%) starts choking in the flow line and equipments. The mixing of molasses with hydrated lime provides a good binding strength; as a result the dust formation is less in comparison to briquettes without molasses.

The process of briquette making can be summarized by following flow chart-
Linz –Donawitz (LD) process

Gas Generation with Dust

Gas Stripped of dust with water

Sludge separation from GAS CLEANING PLANT (GCP) outlet water (250 Microns)

Sludge (soaked with water)

Drying

Sieving

Mixing with binder (for 30-40 minutes)

Storing

Feeding

Briquetting

Curing the briquettes(for 3-5 days)

Transporting to hopper

Weighting bunker

Adding briquettes in LD process

The typical Chemical characteristics of briquettes are as per the following table.

Characteristics of briquettes (% weight)
Total Fe Cao MgO SiO2 P Moisture Shatter Index
(+10mm %)
60-63 10-14 2-5 3-5 0.1-0.2 < 1.5 0.2-0.3

The quality parameters of the briquettes formed in the above process were decided keeping in view the following criteria.
a) Iron content
b) Chemical composition
c) Moisture
d) Shatter Index
The cured briquettes are dispatched to weighting hopper. From hopper the briquettes are transported through conveyor to steel melt shop bunker. The briquettes charges in LD process according to the weight required.

The use of briquettes formed by above process has reduced the iron ore addition in the BOF converter. The reduction in oxygen consumption is another benefit due to the presence high quantity of oxides in the gas cleaning plant (GCP) sludge. It is also beneficial for the good vessel coating. A better coating of the BOF directly results into a longer life of the coating of the converter.

Another major advantage of the briquettes is the high cooling effect of the oxidized by-products of BOF. The dust, scales and sludge from iron- and steel making processes, which usually have high iron content, are in oxidized form. Iron oxide containing material can be used as a substitutive coolant during converter blowing process.
Generally scrap and iron ore are used as coolants. Charging iron oxide to converter may also speed up slag formation in the beginning of the process and decarburization reactions. It is also possible to prevent oxidation of dust in some cases, when the cooling effect decreases. The cooling effect of the briquettes is close to cooling effects of sinter used in secondary cooling. These factors may increase scaffold formation on the inner surface of the flue gas duct. Using by-products as raw material may increase content of some elements in raw steel.

However the present invention to a great extent countered the problems identified as above because the actual trials shown no adverse effect on the product quality and quantity. There was no negative cooling effect in either of the furnace following the mix proportions devised.

Use of briquettes is formed by the above process is advantageous due to the following reasons:
(1) Use of Iron ore as a coolant in the LD process has been eliminated partially, as a result less time is required for slag formation;
(2) Good coating of converter vessel;

The briquettes formed by above process are low cost, in-house produced by-products, which have been found to be valuable recyclable material. The present invention is an industrial process for treating steel making GCP sludge to produce useful briquettes having improved flow rate properties in a recycling system. The briquettes made by the above process are environment friendly and utilized in most economical way.

While the invention has been described in detail with the help of a preferred embodiment, the invention is not limited to the disclosed examples. Other variations can be deducted by those skilled in the art without leaving the scope of protection of the claimed invention.